The present invention relates to a memory which uses a quantum effect and from/into which information can arbitrarily be read/written.
A Josephson device is expected as the device which can achieve both a high speed operation and a low power consumption which cannot be realized by semiconductor devices, and the applications of the Josephson device to memories, logical circuits and sensors are proposed. For example, a memory using the Josephson device stores information in a manner of the presence/absence of a magnetic flux trapped in a superconducting ring or the direction of the magnetic flux. It is assumed that the magnitude of the trapped magnetic flux is limited to integer times of the magnetic flux quantum .phi..sub.0 =h/(2e) (h being the Planck's constant and e being the charge of an electron) and that there is a stable state when a magnetic flux integer times as large as the magnetic flux quantum is trapped in the superconducting ring.
An example of a conventional memory cell using Josephson devices is shown in FIG. 1. A superconducting ring 15 trapping a magnetic flux has a DC superconducting quantum interference device (SQUID) structure in which Josephson devices J1 and J2 are inserted. A word line 11 is connected to the SQUID for supplying a current I.sub.w from the exterior. A write data line 12 and a read data line 13 magnetically coupled with the SQUID are provided adjacent to the ring, and a Josephson junction J3 is inserted in the course of the read data line 13. A superconducting loop current 14 (IL) flows in the superconducting ring 15 so that a magnetic flux is trapped in the superconducting ring 15. A relationship between the magnetic flux .phi. and the potential energy U of the system in the above SQUID structure is shown in FIG. 2. As shown in the figure, a plurality of stable magnetic flux states exist since the potential energy U takes the minimum values at two points of .phi.=0 and .phi.=.phi..sub.0 when the magnetic flux .phi. trapped in the superconducting ring is taken along the abscissa and the potential energy U is taken along the ordinate. Since these states semipermanently endure under a low temperature environment, it is possible to provide a memory by making each state correspond to information to be stored.
This memory cell operates in accordance with a timing chart shown in FIG. 3.
A bias current is flown in the write data line so that a bias magnetic field is applied to the SQUID. In a case where data "0" is to be written into the memory cell, the word line current I.sub.w is brought into a high level and a current larger than a critical current Ic, by which the Josephson junction is transited into a finite-voltage state, and smaller than 2Ic is flown in the word line. When a write data line current ID.sub.w is turned to 0, the devices J1 and J2 are both brought into zero-voltage states so that the current I.sub.w flowing through the word line is bisected. As a result, the magnetic flux passing through the SQUID becomes zero to realize a state of "0". On the other hand, when the write data line current ID.sub.w is turned to not 0 but a high level, a magnetic field produced by ID.sub.w increases the critical current of on of the devices J1 and J2 and decreases the critical current of the other device. As a result, only one of the devices J1 and J2 takes a nonzero-voltage state and the current I.sub.w flows into only the other device of a zero-voltage state so that data "1" is written. Even if ID.sub.w and I.sub.w are thereafter restored to non-write states, a superconducting loop current IL is maintained so that the magnetic flux .phi..sub. o is trapped in the loop.
When data is to be read, the current I.sub.w is brought into the high level in the same manner as at the time of write of data and a current ID.sub.R is flown in the read data line. In a case where data "1" is stored in the memory cell, a magnetic field produced by the current I.sub.w and a magnetic field produced by the current IL interact to enhance each other, thereby decreasing a critical current of the Josephson device J3. As a result, the device J3 exhibits a transition to a finite-voltage state and V.sub.out becomes high. In a case where the current IL is 0, the current ID.sub.R is smaller than the critical current and the output V.sub.out remains in a zero-voltage state.
In principle, a response on the order of subpicosecond is expected for a Josephson device when it is used as a single or discrete device. However, in a case where Josephson devices are used to form a system such as a logical circuit or an integrated memory circuit, there has not yet been developed a system which exceeds the limit of the response characteristic of a semiconductor integrated circuit. A further reduction in the size of a Josephson device is needed in order to further improve an operating speed.
However, it is pointed out that a new quantum effect having not hitherto been expected may occur as the area of the junction is reduced (see A. J. Leggett and Anupam Garg, Physical Review Letters, Vol. 54, pp. 857-820, March 1985). Namely, when the capacitance of a Josephson junction decreases as the junction area is reduced, an electrostatic energy possessed by a Cooper pair becomes innegligible as compared with the pairing energy of a Cooper pair. This condition is given by e.sup.2 /C.about..DELTA. where .DELTA. is an energy gap of the superconductor and C is the capacitance of the Josephson junction. The magnetic flux state of this SQUID is not localized at the minimum potential point but has a certain broadening based on the uncertainty principle, as shown in FIG. 4. As shown in FIG. 2, there are two states for magnetic flux allowed to settle and transition occurs between these two states due to quantum mechanical fluctuation. Such transition may be understood as tunnel phenominum. As a result, the magnetic flux trapped in a superconducting ring decays due to the tunnel effect with the reduction of the junction area, so that the flux state, in which the magnetic flux integer times as large as the flux quantum is trapped in the superconducting ring, becomes unstable. Therefore, the conventional Josephson memory as shown in FIG. 1 reaches a limit of a storage or memory operation.